The present invention relates to an ink film thickness distribution forming method and apparatus for forming an ink film thickness distribution in an ink roller group in an ink supply apparatus.
FIG. 11 shows the main part of an inker (ink supply apparatus) in a printing unit of each color in a web offset printing press. In FIG. 11, the inker includes an ink fountain 1, an ink 2 stored in the ink fountain 1, an ink fountain roller 3, a plurality of ink fountain keys 4 (4-1 to 4-n) juxtaposed in the axial direction of the ink fountain roller 3, an ink ductor roller 5, an ink roller group 6, a printing plate 7, and a plate cylinder 8 on which the printing plate 7 is mounted. An image is printed on the printing plate 7.
In the ink supply apparatus, the ink 2 in the ink fountain 1 is supplied to the ink fountain roller 3 by adjusting the opening degrees of the ink fountain keys 4-1 to 4-n. The ink supplied to the ink fountain roller 3 is supplied to the printing plate 7 via the ink roller group 6 by the ink feed operation of the ink ductor roller 5. Note that ink form rollers 6-1 to 6-4 in contact with the printing plate 7 are arranged at the end of the ink flow path of the ink roller group 6.
When switching a print job in the ink supply apparatus, that is, when replacing the printing plate 7 for a preceding print job with a printing plate 7′ for the next print job, the opening degrees of the ink fountain keys 4-1 to 4-n, the rotation amount of the ink fountain roller 3, and the like are changed to values corresponding to an image on the printing plate 7′ for the next print job. The ink 2 in the ink fountain 1 is supplied to the replaced printing plate 7′ via the ink roller group 6. In this case, test printing is performed before final printing to adjust the ink supply amount, obtaining a satisfactory color tone. As a result, a desired ink film thickness distribution (gradient of the ink film thickness) is formed in the ink roller group 6.
However, in a conventional ink supply apparatus, when the printing plate 7 is replaced with the printing plate 7′ to execute the next print job, an ink film thickness distribution corresponding to the printing plate 7 for the preceding print job remains in the ink roller group 6. In this case, the ink film thickness distribution corresponding to the printing plate 7 for the preceding print job needs to be gradually changed to an ink film thickness distribution corresponding o the printing plate 7′ for the next print job. Adjustment of the ink supply amount and test printing are required excessively until a satisfactory color tone is obtained. This causes problems such as “increase in pre-printing preparation time”, “increase in work load”, “waste of printing materials”, “decrease in production efficiency”, and “increase in cost”.
To reduce adjustment of the ink supply amount and the test printing count until a satisfactory color tone is obtained, there have been proposed ink film thickness control methods disclosed in Japanese Patent Laid-Open Nos. 10-16193 (literature 1) and Japanese Patent Laid-Open No. 11-188844 (literature 2).
[Literature 1 (Ink-decrease+Pre-inking 2)]
In the ink film thickness control method described in literature 1, when switching a print job, the ink feed operation of the ink ductor roller 5 is stopped. While the printing plate 7 for the preceding print job is kept mounted, the printing press is operated to print a predetermined number of sheets (blank sheet printing), decreasing ink in the ink supply apparatus (ink-decrease). A minimum ink film thickness distribution Ma (see FIG. 12A) which thins from the upstream side to downstream side of the ink roller group 6 and is required during printing, that is, an ink film thickness distribution Ma corresponding to an image-free portion of the printing plate 7 remains (ink removing).
Then, the opening degrees of the ink fountain keys 4-1 to 4-n, the rotation amount of the ink fountain roller 3, and the like are set to values corresponding to an image on the printing plate 7′ for the next print job. The printing press is operated to perform the ink feed operation of the ink ductor roller 5 by a predetermined number of times. An ink film thickness distribution Mb (see FIG. 12B) corresponding to the image on the printing plate 7′ for the next print job is superposed on the minimum ink film thickness distribution Ma which remains in the ink roller group 6 and is required during printing (pre-inking 2).
[Literature 2 (Ink Return to Fountain+Pre-inking 1)]
In the ink film thickness control method described in literature 2, when switching a print job, the opening ratios of the ink fountain keys 4-1 to 4-n are set to 0. In this state, the ink feed operation of the ink ductor roller 5 is performed by a predetermined number of times, returning all ink remaining in the ink roller group 6 to the ink fountain 1 (“ink return to fountain”). As a result, each roller in the ink roller group 6 does not hold any ink.
The opening degrees of the ink fountain keys 4-1 to 4-n are set to a predetermined value (e.g., 50%), and the rotation amount of the ink fountain roller 3 is set to a predetermined value (e.g., 50%). Then, the ink feed operation of the ink ductor roller 5 is performed by a predetermined number of times, forming a minimum ink film thickness distribution Ma (see FIG. 12A) required during printing in the ink roller group 6 (first step of pre-inking 1).
The opening degrees of the ink fountain keys 4-1 to 4-n, the rotation amount of the ink fountain roller 3, and the like are set to values corresponding to the image on the printing plate 7′ for the next print job. The printing press is operated to perform the ink feed operation of the ink ductor roller 5 by a predetermined number of times. An ink film thickness distribution Mb (see FIG. 12B) corresponding to the image on the printing plate 7′ for the next print job is superposed on the minimum ink film thickness distribution Ma which is formed in the ink roller group 6 and required during printing (second step of pre-inking 1).
However, the ink film thickness control method described in literature 1 (ink-decrease+pre-inking 2) wastes sheets because blank sheet printing is executed when leaving the ink film thickness distribution Ma on the ink roller group 6.
The ink film thickness control method described in literature 2 (“ink return to fountain”+pre-inking 1) takes time because all ink on the ink roller group 6 is returned to the ink fountain 1 and an ink film thickness distribution (Ma+Mb) corresponding to the image on the printing plate 7′ for the next print job is formed from 0. In this method, emulsified ink (ink kneaded with damping water) is returned to the ink fountain 1. A printing trouble may occur, wasting printing materials.